PowerPoint Presentation - University of Iowa Astrophysics by yurtgc548

VIEWS: 6 PAGES: 31

									             Gamma-Ray Bursts
• Energy problem and beaming *
• Mergers versus collapsars
• GRB host galaxies and locations within galaxy
• Supernova connection
• Fireball model *
• Swift
• Afterglows of short bursts
              Energy Problem
• GRB 990123 required a total energy, if
  isotropic, of 3.41054 erg = 1.9 M c2.
• GRB energy source is almost certainly
  gravitational – need few M collapsed into
  region not more than 100 km across.
• Energy density U = T4/c
     T ~ 1012 K kT ~ 100 MeV
    This is high enough to produce e+e- pairs.
                       Fireball
• Consider pure energy confined within a sphere,
  describe with E, R, T (Goodman 1986)
• Radius of sphere R = (3E/11T4)1/3
• Optical depth from center to edge
                                      1/ 3
    T                   15    E   kT 
          4                                          5/3

 
    kT       T R  4 10  51
                              10 erg   1 MeV 
                                       
                                            
• Edge of sphere (photosphere) will expand at a speed
  close to c as long as kT > mec2
• If baryons are added, most energy goes into
  accelerating baryons to ~ E/Mc2
                       Fireball
• Optical depth from center to edge for burst which
  varies over time scale t with a sepctrum such that a
  fraction fp of the photon pairs can pair produce.

                                            2
                    E  T 
          10 f p  50
               11
                    10 erg  10 ms 
                            
                                  
• Very high optical depth is inconsistent with non-
  thermal spectrum at high energies
            Relativistic outflow
• In a relativistic outflow, the observed photon energy is
  a factor  (= Lorentz factor of bulk motion) higher
  than the photon energy in the rest frame. For a
  spectrum with an energy index  this reduces the
  number of photon pairs above the electron-positron
  threshold by –2
• Also the size of the emitting area can be larger by a
  factor 2
                              ct        R
      v  c 1    ct  vt  2  t 
                   2

                             2        2c 2
• Need  ~ 100 to solve the problem.
            Evidence for Jet
Afterglow of GRB 990123 shows a break
                   Beaming
Because of relativistic motion, radiation is beamed
with an opening angle ~ 1/
Therefore, observer can see only a limited piece of an
expanding shell



                                    Observer


                1/ 
               At Early time:  1  

                                           = jet angle

     1

                Area visible to an observer = (R/)2
R

                 At Late time:  1  



 1
           


                   Area visible to an observer = (R)2
    R
Monochromatic Jet Break
                  Jet Breaks
• Jet opening angle is related to time at which
  break in light curve occurs




• Beaming fraction is determined by jet opening
  angle = 1 – cos  2/2
• Energy required is reduced by a factor 2/2
Jet Energy




Frail et al. 2001
                Burst Models

•   Collapsing WDs
•   Stars Accreting on AGN
•   White Holes
•   Cosmic Strings
•   Black Hole Accretion Disks
    I) Binary Mergers II) Collapsing Stars
     Mergers
Binaries must evolve
before merger and binaries
have non-zero speeds due
to kicks in compact object
formation.
Thus, GRBs can occur in
outskirts of or even far
from host galaxy.
          Massive Star Collapse




 Beamed Explosion, accompanying supernova-like explosion,
GRBs should be associated with young, massive stars.
 Host
Galaxies



                           Holland 2001


           Hosts are similar to star-forming
           galaxies at similar redshifts.
           High star formation rates.
Location of GRB within Host
Location of
                            Distribution
GRB within                  Follows
   Host                     Stellar
                            Distribution

The environments of
GRBs show higher gas
densities, higher
metallicities, and higher
dust content than
random locations in
host galaxies.
Suggests that GRBs
occur in star forming
regions.
               GRB Locations
• GRB hosts are star-forming galaxies
• GRBs trace the stellar distribution (in distance
  from galaxy center)
• GRBs occur in dense environments (probably
  star forming regions)

• Suggests collapsar model over merger model
          Supernova connection
SN 1998bw was found in the 8’ error circle of GRB
980425 in observations made 2.5 days after the burst.
A slowly decaying X-ray source was subsequently found
in the same galaxy (z = 0.0085) and identified with the
GRB.
However, the GRB was very underluminous and the SN
was very usual with parculiar line emission (no H, no He,
no Si at 615 nm.
Radio emission a few days after GRB indicated
relativistic outflow with energy ~ 31050 erg.
Thought to be oddball GRB and SN.
GRB030329 and SN 2003dh
             Clear spectroscopic
             signature of a SN, broad
             emission lines, found after
             decay of afterglow of
             GRB030329.
             “Smoking gun” linking
             GRBs and SNe.
SN 2003dh versus SN 1998bw
SN Bumps
              GRB - Supernova




Only a tiny fraction of SN are observed to be GRBs
GRB 060218 = SN 2006aj
                     Fireball Model




Initial event accelerates baryons in bulk
Later on, internal shocks re-accelerate particles
produce GRB
Even later, external shocks produce afterglow
                      GRB 990123




990123 reached 9th magnitude for a few moments!
First optical GRB afterglow detected simultaneously
       Internal-External Shock Model
                                         External Shocks
                 Internal Shocks
                                                    ISM
Central Engine




                           R  1014 cm            R  1016 cm

                   GRB                       Afterglow
                                                                3
            Burst (as Jet) Properties



 3. Baryonic mass content of the jet ~ 2x10-7   6x10-6 Mo




Baryon mass is ~ 10-5 M
Jet opening angle means that we observe only one of each 1000
GRBs in the Universe, most are pointed away.
The means that GRB rate is about 1% of SN rate.
                                           Swift
                                       BAT – CZT detector with
                                       5200 cm2 area sensitive in
                                       15-150 keV band.
                                       Coded aperture imaging of
                                       1.4 steradian field with 4
                                       arcmin resolution suing
                                       32768 pixels.

After detecting a burst, Swift autonomously repoints bringing the
burst into view of the XRT and UVOT, often within 90 seconds.
XRT – focusing X-ray telescope in 0.5-6 keV band, 2.5 arcsecond
source location accuracy.
UVOT – focusing UV/optical telescope.
                      Swift Results
• Launched in 2004.
• Detects about 100 bursts/year
• More afterglow detections than all previous satellites combined
• GRB with redshift of z = 6.29
• Average redshift = 2.7 compared to pre-Swift <z> = 1.2
•
• Expect 40 GRB with z > 5 and 4 with z > 8.
         Afterglow of short GRB




GRB 050509b associated with elliptical galaxy.
HETE-II GRB 050724 also associated with elliptical.

								
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